OH in Rutile: An Oxygen and Water Barometer

俯冲岩石圈的脱水引起俯冲带上方地幔楔体的氧化和部分熔融，水在高压矿物相中以羟基形式的储存可能是水向地幔转移的重要机制。最近，人们对非常早期（4.3 Ga）地球上可能存在水圈产生了浓厚的兴趣，强调需要更多地了解岩石的流体组成和氧逸度，这可以提供有关早期陆地氧化状态和水循环的重要信息。氧逸度和水活度很难测量或推断，除非能找到共存的氧或水缓冲矿物组合。该项目将通过实验研究金红石中OH浓度与施加的氧逸度、水活度、三铁含量、温度和压力之间的关系，并将使用热力学模型将这些结果应用于地质系统，其中限制氧逸度和流体成分很重要，但很难用其他方法进行评估。该技术基于金红石（TiO2）中的OH浓度测量，金红石（TiO2）是一种常见的辅助矿物，名义上是无水的，但可以在其结构中加入大量的羟基。在纯金红石中，H+通过电荷耦合将Ti4+还原为Ti3+，以化学计量的方式加入到结构中（导致颜色从淡黄色变为深蓝）。氢在金红石中的扩散实验将导致平衡边界条件的量化，以及名义上无水矿物中氧化还原驱动和内在缺陷驱动的氢损失速率。一个新的扩散模型将从这些数据中构建，以解决先前在名义无水矿物中进行的氢扩散实验和自然样品中OH测量的看似矛盾的结果。金红石氧水晴雨表一旦研制成功，将在地球化学和岩石学领域具有广泛的应用前景。即使在缺乏含水矿物或传统的氧缓冲矿物组合的系统中，也可以确定水的活度或氧逸度。拟议中的研究将允许PI，一位早期职业女性科学家，在学术环境中进行积极的研究。这项资助将在两个暑假期间支持本科生的研究援助。

Dehydration of the subducting lithosphere induces oxidation and partial melting in the mantle wedge above subduction zones, and storage of water in the form of hydroxyl in high-pressure mineral phases may be an important mechanism for transfer of water to the mantle. Recently there has been much interest in the possible existence of a hydrosphere on the very early (4.3 Ga) Earth, highlighting the need to learn more about the fluid composition and oxygen fugacity of rocks that can provide important information about the early terrestrial oxidation state and water cycle. Oxygen fugacity and water activity are difficult to measure or infer unless one can find coexisting oxygen- or water-buffering mineral assemblages. This project will experimentally investigate the relationship between OH concentrations in rutile and the imposed oxygen fugacity, water activity, ferric iron content, temperature, and pressure, and will use a thermodynamic model to apply these results to geologic systems for which constraining oxygen fugacity and fluid composition is important, but has been difficult to evaluate with other methods. The technique is based on OH concentration measurements in rutile (TiO2), a commonly occurring accessory mineral that is nominally anhydrous but can incorporate a significant amount of hydroxyl into its structure. In pure rutile, H+ is stoichiometrically incorporated into the structure via charge-coupled reduction of Ti4+ to Ti3+ (resulting in a change in color from pale yellow to deep blue). Diffusion experiments of hydrogen in rutile will result in quantification of equilibrium boundary conditions and rates of redox-driven and intrinsic defect-driven loss of hydrogen in nominally anhydrous minerals. A new diffusion model will be constructed from this data to resolve the seemingly contradictory results from previous hydrogen diffusion experiments in nominally anhydrous minerals and measurements of OH in natural samples. Once developed, the rutile oxygen and water barometer should have widespread applications in geochemistry and petrology. Water activity or oxygen fugacity will be able to be determined even in systems lacking hydrous minerals or traditional oxygen buffering mineral assemblages. The proposed research will allow the PI, an early-career female scientist, to carry on active research in an academic setting. This grant will support undergraduate research aids over two summers.